1. Description of Related Art
Integrated circuits typically include various active and passive circuit elements which have been integrated into a piece of semiconductor material, often referred to as a die. The die may, in turn, be encapsulated into a package, which often includes a ceramic or plastic substrate although other materials may be used. These packages are usually attached to a printed circuit board, often by connecting pins arranged along the periphery of the package. In this manner, an electronic system can be assembled by connecting various integrated circuit packages to a printed circuit board.
In addition to mechanically connecting the integrated circuit package to the printed circuit board, the connecting pins also typically provide separate electrical connection terminals between the printed circuit board and the various inputs and outputs of the integrated circuit within the package. To increase the number of connection terminals, other package designs have been utilized. For example, in the pin grid array (PGA), ball grid array (BGA) and land grid array (LGA) packages, a large number of input/output (I/O) connection terminals are disposed in a two dimensional array over a substantial portion of a major surface of the package.
To increase space utilization, two or more integrated circuit dies may be attached to a printed circuit board in a stacked arrangement. The dies may be interconnected in a die-to-die stacked arrangement. Alternatively, each die may be placed in a package and the two packages may be stacked in a package-to-package arrangement. FIG. 1a shows an exploded view of one such known package-to-package stack indicated generally at 10. The stack 10 includes a first integrated circuit package 12, and a second integrated circuit package 14 which are physically and electrically connected together as shown in FIG. 1b using an interposer 16.
The integrated circuit package 12 includes a package substrate 20 on which an integrated circuit die 22 is mechanically and electrically connected by a plurality of solder bumps 24. Similarly, the integrated circuit package 14 includes a package substrate 26 to which an integrated circuit die 28 is mechanically and electrically connected by a plurality of solder bumps 30. Other electrical connectors including wires may be used in place of or in addition to the solder bumps 24, 30. The package substrates 20, 26 may have both internal and exterior conductors which are electrically connected to the solder bumps 24, 30 or to contact pads on the dies 22, 28.
The dies 22, 28 may be encapsulated in a polymer such as an epoxy layer 32 depicted for the die 28. The inputs and outputs of the stack 10 may be electrically connected to a printed circuit board using connection pins, solder bumps or other connection terminals.
As best seen in FIG. 1c, the interposer 16 includes a generally rectangular ring-shaped frame 34 which may be constructed of a dielectric material such as plastic or ceramic, for example. The frame 34 has a plurality of apertures distributed about its periphery into which plugs 36 may be punched into the frame apertures and secured therein in an interference fit. The plugs 36 are typically formed of an electrically conductive material such as copper and may each be bonded to aligned contact pads 40 and 42 of the package substrates 20 and 26, respectively, as shown in FIG. 1b. The copper plugs 36 may be bonded to the contact pads 40, 42 of the package substrates 20, 26 using stencil printed solder or other materials including electroplated solder, ink jet solder or adhesives or using other bonding techniques including thermocompression and thermosonic joining.
Each plug 36 can provide a separate electrical interconnection between the packages 12 and 14. The center to center spacing or “pitch” between adjacent force fit plugs 36 may be as low as 300 microns, in some applications. Interposers may be used to provide die-to-die or die-to-package substrate interconnections.